Saturday, October 13, 2007

Sea of Noise

There has not much been much change to the spatial distribution of global tropical SSTs since my 8 October posting. A basin wide cold event appears to be in the process of maturing across the equatorial Pacific Ocean basin (165E -South America) with anomalies ~minus 3C from 120-140W extending to around 200m deep. The warm Indian Ocean -cool Indonesia -warm west central Pacific Ocean pattern of Eastern Hemisphere SSTAs persists with totals in excess of 30C ~0-10N/150-160E (merging with the warm Pacific Ocean horseshoe). Other SST details can be found from the links below in the Appendix and my previous posting.

The gist is that the global circulation remains strongly coupled to La-Nina in terms of circulation response. Given the complex dynamics of forcing-response-feedback global circulation-tropical convection evolutions since around 1 December 2006, perhaps it is the SSTs that have coupled to the atmosphere particularly starting around 1 September 2007. Reiterating, I did not think we would see a basin wide cold event a couple of months ago. In any case, this coupling has been and continues to modulate synoptic variability globally including the USA. From the GSDM (and more generally subseasonal) framework the latter is scientifically defensible; I strongly disagree with statements to the contrary from anywhere. The latter are from a perspective of, for example, 3-month composites of temperature and precipitation anomalies which will average out important subseasonal weather events.

Per ESRL/PSD plots, global relative atmospheric angular momentum (AAM) has decreased during the past week to roughly 3 AMUs below the R1 data climatology. In addition to the well established anomalously strong zonal mean easterly wind flow throughout the subtropical atmospheres (~25N and 25S with magnitudes ~10m/s at 200mb), anomalous easterly wind flow is developing both in the tropics and higher latitudes. The former is tied to the tropical convective forcing and the latter is a response to extremely intense midlatitude anticyclones (anomalies in excess of 40m/s at times) amplifying into the global Polar Regions. In fact, a rare sudden stratospheric warming of the austral (spring) stratosphere may be in progress as a response (also discussed 8 October). Both AAM tendency and the surface torques have become slightly positive; however, the Global Wind Oscillation (GWO) remains solidly in GSDM Stage 1.

There has been a significant subseasonal perturbation onto our GSDM Stage 1 La-Nina basic state during the past week. As seen from animations of daily mean upper tropospheric vector wind anomalies and time-longitude sections of velocity potential (divergent part of the total velocity field), persistent Rossby wave energy dispersions arcing from the twin subtropical anticyclones ~120E through both hemispheres led to an increase of divergence centered around the Americas. Strong convection (OLRA ~ minus 50-90 W/m**2) developed from the East Pacific ITCZ into the Caribbean and the Amazons of South America. Weak anomalous twin subtropical anticyclones have formed as a response ~60-90W leading to Western Hemisphere equatorial easterlies.

Current full disk satellite imagery shows tropical convection enhanced around the Americas. There is also renewed tropical forcing around 0/80E, with strong suppression across Indonesia into the west Pacific. A week ago I thought there was some observational evidence that a MJO may be in the early stages of development. This notion is still not unreasonable. However, I did not think a MJO might develop as the result of an extratropical feedback forcing a Western Hemisphere tropical convective dynamical signal, currently ~60-70W. These dynamical signals typically propagate ~15m/s (roughly 12 deg long/day) when they are in the Western Hemisphere. Simple extrapolation would put this signal solidly across the Indian Ocean in roughly 8-10 days.

Even though we are in a stationary GSDM Stage 1 base state, my confidence for any week 1-3 forecast across the PNA sector is very low. In addition to getting “surprised” (RMM WH2004 phase space plots did suggest this possibility; these kind of chain of events have occurred in the past such as December 2005), seasonal transition into boreal cold season is also adding difficulty. It also seems like any predictive information I have offered lately has been poor. This only confirms once again we are dealing with a dynamical system characterized by a sea of noise that does not like to be predicted. Tools such as the GSDM, GWO quasi-phase space plot and risk assessment plots of subseasonal variability, along with rigorous daily monitoring, help to facilitate “forecasts of opportunity (strong coherent signals above the background noise)” especially when numerical and other statistical tools are also struggling. Like it or not, that is the way it is (the truth)!!

My feeling is to expect a reinvigoration of the tropical convective forcing across the equatorial Eastern Hemisphere from ~60-120E by the end of week 2. The La-Nina enhanced westerlies ~40N have been coming south, regionally recently slamming the USA west coast with several strong troughs. If the Indian Ocean convection becomes quite robust, and that is probable given our base state (which may further strengthen La-Nina), the North Pacific jet would be expected to collapse. In fact, a full-latitude ridge into Alaska may develop initially along the west coast then retrograde to ~140-150W by week 3.

For the USA, this suggests about 1 more week of an unseasonably active weather pattern followed by quieter conditions week 2 (models suggest this). If this were about a month or so later I would be expressing a concern for a cold/stormy regime (with Arctic air) across the western half of the country starting week 3. Should a decent MJO develop across the Indian Ocean along with the GWO, a “classic” GSDM Stage 1 cold season regime for North America may start to evolve by that time (~ 28 October-4 November) in spite of the seasonal cycle. My speculation for this upcoming boreal winter season stated in past discussions remains the same.

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Anticyclonic wave breaking reigns supreme across the North Atlantic. The spirit of “Gabrielle” may still live.

Heavy rainfall may become a concern for portions of equatorial Africa week 1 then into the Indian Ocean weeks 2-3 per above. Tropical cyclone hazards may actually become more probable for the South Indian Ocean (one already did try) than Bay of Bengal weeks 2-3. The west central Pacific will always be the wild card. One would expect general suppression from Indonesia into the west Pacific Ocean weeks 1-2, with activity increasing especially along the equator by week 3. However, who really knows the timing at this point?

Appendix

An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion ~19-20 October.